(1) Field of the Invention
The present invention relates to fixing devices and image forming apparatuses that are provided with a fixing member and that thermally fix an unfixed image onto a recording sheet, the fixing member being heated by electromagnetic induction and including a magnetic shunt alloy layer having a Curie temperature higher than a fixing temperature. More particularly, the present invention relates to technology for preventing the occurrence of uneven gloss and uneven fixing of a fixed image.
(2) Description of Related Art
An electromagnetic induction heating type fixing device is now commonly used as a fixing device in image forming apparatuses such as printers or copiers.
In such an electromagnetic induction heating type fixing device, high-frequency current is passed through an excitation coil to produce an alternating magnetic field, thus producing an eddy current in the heating layer of the fixing member, which is in the shape of a belt, resulting in Joule heating. This allows for a reduction in the heat capacity of the fixing member, thus offering advantages such as energy saving and a shorter time for warming up as compared to an image forming apparatus that has a fixing device using a heater.
The fixing member has a small heat capacity, however, making it easy for the temperature of the sheet passing region to lower due to the passing of a recording sheet. In order to maintain the temperature of the sheet passing region at the fixing temperature, it is necessary to continue heating the fixing member for the duration of heat fixing operations.
As a result, when many sheets pass continually through the nip, the temperature of non-sheet passing regions that are not deprived of heat by the recording sheets becomes extremely high. Such a high temperature causes the problem of deterioration of the fixing member, resulting in a shorter lifetime.
To address this problem, Japanese Patent Application Publication No. 2008-70757, for example, discloses a fixing device that heats a fixing member using a heat generating element that includes a magnetic shunt alloy. The magnetic shunt alloy has a Curie temperature set to be higher than the fixing temperature yet lower than the temperature limit of the fixing member.
The fixing device disclosed in this patent application publication uses a magnetic shunt alloy as the heat generating element. When the temperature of the heat generating element in the non-sheet passing regions rises to the Curie temperature, the heat generating element transitions from being ferromagnetic to being paramagnetic. The magnetic flux density flowing through this portion suddenly decreases, thus reducing the amount of heat. The temperature of the non-sheet passing regions of the fixing member is thus prevented from rising excessively.
The inventors discovered, however, that when using a magnetic shunt alloy in order to prevent an abnormal rise in temperature in the non-sheet passing regions, as in the fixing device of the above patent application publication, a reduction in fixity and a difference in glossiness in a portion of a recording sheet as compared to other portions occur when the recording sheet is passed just when the magnetic shunt alloy reaches the Curie temperature. Such uneven fixing and uneven gloss occur in a strip extending in the direction of width of the recording sheet (the direction orthogonal to the sheet passing direction).
Normally, temperature control of the fixing device is performed using feedback control in a power control unit, whereby the power supplied to the excitation coil is determined based on the surface temperature of the sheet passing region as detected by a temperature sensor, and the determined power is stably supplied to the excitation coil. When the magnetic shunt alloy in the non-sheet passing regions reaches the Curie temperature, as described above, the permeability suddenly changes, causing the inductance of the excitation coil to drastically change, which greatly lowers the output of the excitation coil.
The power control unit then performs feedback control in an attempt to recover the output of the excitation coil. Due to the resulting time lag, the temperature of a portion of the fixing member necessarily decreases, however, leading to an uneven temperature. This is thought to cause uneven fixing or uneven gloss along a strip in the recording sheet.